A flash memory device is a nonvolatile memory medium. Stored data is not lost even when the power supply is turned off. Flash memory has a relatively high data processing speed for recording, reading, and erasing. Accordingly, the flash memory device may be used for a BIOS in a PC, and for storing data in a set-top box, a printer, and a network server, etc. Flash memory may also be used in digital cameras and cellular phones, etc.
In a flash memory device, a stack gate type semiconductor device using a floating gate, and a silicon-oxide-nitride-oxide silicon (SONOS) structure are used. Flash memory devices can be competitive only when unit cells are concentrated on a narrow area, using a common source line rather than a separate contact on each source. A common source line is formed to be larger than a bit line. This affects the formation of adjacent bit lines, due to lines having irregular sizes, thereby creating a difficulty in forming a uniform pattern.